The invention relates to an organic light-emitting diode (OLED) having an improved lifetime and improved transport of negative charge carriers.
OLEDs based on semiconductor material are known having, for example, a matrix of polyarylenevinylene or polyparaphenylene (in particular polyfluorene and/or polyspirofluorene), in which other chromophores and/or triarylamine derivatives are polymerized in addition to these components or are introduced in the matrix to form of blends. As a rule, chromophores produce strong long-wavelength emission bands, i.e., essentially green or red, in the resulting electroluminescence spectra. The triarylamine units typically have little influence on the emission spectrum of the organic light-emitting diodes and serve primarily to transport positive charges over the thoroughly conjugated polymer chain and/or improve the oxidation stability of the organic semiconductor material.
With the organic semiconductor materials described above for active layers of an OLED, it is currently possible to cover a large range of possible CIE color coordinates and achieve relatively high luminous densities and efficiencies that would be relevant for commercial applications. However, the useful life of polymer OLEDs is still too low to be successful in the marketplace. This is true in particular of the shorter wavelength light-emitting diodes, i.e., the blue- and white-emitting LEDs and/or displays such as the so-called passive matrix displays.
A fundamental problem with the useful life of polymer OLEDs is the resistance to reduction of the organic semiconductor material. This problem is crucial because in an organic semiconductor material, charge transport is accomplished via individual oxidation and reduction reactions, where a subunit involved in charge transport must typically be oxidized or reduced several billion times during the useful life of the component. Irreversible chemical degradation during such a process results in a worsening of charge transport properties and at the same time a reduction in luminous density.
The object of the present invention is therefore to create an organic light-emitting diode using a semiconducting organic material having increased redox stability so that it will have a longer useful life.